diff --git a/examples/companion_radio/main.cpp b/examples/companion_radio/main.cpp
index 151f90c3..912f9593 100644
--- a/examples/companion_radio/main.cpp
+++ b/examples/companion_radio/main.cpp
@@ -438,13 +438,16 @@ protected:
     return false;
   }
 
-  void queueMessage(const ContactInfo& from, uint8_t txt_type, uint8_t path_len, uint32_t sender_timestamp, const char *text) {
+  void queueMessage(const ContactInfo& from, uint8_t txt_type, uint8_t path_len, uint32_t sender_timestamp, const uint8_t* extra, int extra_len, const char *text) {
     int i = 0;
     out_frame[i++] = RESP_CODE_CONTACT_MSG_RECV;
     memcpy(&out_frame[i], from.id.pub_key, 6); i += 6;  // just 6-byte prefix
     out_frame[i++] = path_len;
     out_frame[i++] = txt_type;
     memcpy(&out_frame[i], &sender_timestamp, 4); i += 4;
+    if (extra_len > 0) {
+      memcpy(&out_frame[i], extra, extra_len); i += extra_len;
+    }
     int tlen = strlen(text);   // TODO: UTF-8 ??
     if (i + tlen > MAX_FRAME_SIZE) {
       tlen = MAX_FRAME_SIZE - i;
@@ -462,11 +465,16 @@ protected:
   }
 
   void onMessageRecv(const ContactInfo& from, uint8_t path_len, uint32_t sender_timestamp, const char *text) override {
-    queueMessage(from, TXT_TYPE_PLAIN, path_len, sender_timestamp, text);
+    queueMessage(from, TXT_TYPE_PLAIN, path_len, sender_timestamp, NULL, 0, text);
   }
 
   void onCommandDataRecv(const ContactInfo& from, uint8_t path_len, uint32_t sender_timestamp, const char *text) override {
-    queueMessage(from, TXT_TYPE_CLI_DATA, path_len, sender_timestamp, text);
+    queueMessage(from, TXT_TYPE_CLI_DATA, path_len, sender_timestamp, NULL, 0, text);
+  }
+
+  void onSignedMessageRecv(const ContactInfo& from, uint8_t path_len, uint32_t sender_timestamp, const uint8_t *sender_prefix, const char *text) override {
+    saveContacts();   // from.sync_since change needs to be persisted
+    queueMessage(from, TXT_TYPE_SIGNED_PLAIN, path_len, sender_timestamp, sender_prefix, 4, text);
   }
 
   void onChannelMessageRecv(const mesh::GroupChannel& channel, int in_path_len, uint32_t timestamp, const char *text) override {
diff --git a/examples/simple_secure_chat/main.cpp b/examples/simple_secure_chat/main.cpp
index ef2432fd..9ab8bfa2 100644
--- a/examples/simple_secure_chat/main.cpp
+++ b/examples/simple_secure_chat/main.cpp
@@ -261,6 +261,8 @@ protected:
 
   void onCommandDataRecv(const ContactInfo& from, uint8_t path_len, uint32_t sender_timestamp, const char *text) override {
   }
+  void onSignedMessageRecv(const ContactInfo& from, uint8_t path_len, uint32_t sender_timestamp, const uint8_t *sender_prefix, const char *text) override {
+  }
 
   void onChannelMessageRecv(const mesh::GroupChannel& channel, int in_path_len, uint32_t timestamp, const char *text) override {
     if (in_path_len < 0) {
diff --git a/src/helpers/BaseChatMesh.cpp b/src/helpers/BaseChatMesh.cpp
index 410e27fe..8b494ef4 100644
--- a/src/helpers/BaseChatMesh.cpp
+++ b/src/helpers/BaseChatMesh.cpp
@@ -136,7 +136,29 @@ void BaseChatMesh::onPeerDataRecv(mesh::Packet* packet, uint8_t type, int sender
         if (path) sendFlood(path);
       }
     } else if (flags == TXT_TYPE_SIGNED_PLAIN) {
-      // TODO
+      if (timestamp > from.sync_since) {  // make sure 'sync_since' is up-to-date
+        from.sync_since = timestamp;
+      }
+      onSignedMessageRecv(from, packet->isRouteFlood() ? packet->path_len : 0xFF, timestamp, &data[5], (const char *) &data[9]);  // let UI know
+
+      uint32_t ack_hash;    // calc truncated hash of the message timestamp + text + sender pub_key, to prove to sender that we got it
+      mesh::Utils::sha256((uint8_t *) &ack_hash, 4, data, 9 + strlen((char *)&data[9]), from.id.pub_key, PUB_KEY_SIZE);
+
+      if (packet->isRouteFlood()) {
+        // let this sender know path TO here, so they can use sendDirect(), and ALSO encode the ACK
+        mesh::Packet* path = createPathReturn(from.id, secret, packet->path, packet->path_len,
+                                                PAYLOAD_TYPE_ACK, (uint8_t *) &ack_hash, 4);
+        if (path) sendFlood(path);
+      } else {
+        mesh::Packet* ack = createAck(ack_hash);
+        if (ack) {
+          if (from.out_path_len < 0) {
+            sendFlood(ack);
+          } else {
+            sendDirect(ack, from.out_path, from.out_path_len);
+          }
+        }
+      }
     } else {
       MESH_DEBUG_PRINTLN("onPeerDataRecv: unsupported message type: %u", (uint32_t) flags);
     }
diff --git a/src/helpers/BaseChatMesh.h b/src/helpers/BaseChatMesh.h
index a809c48c..afc75bbe 100644
--- a/src/helpers/BaseChatMesh.h
+++ b/src/helpers/BaseChatMesh.h
@@ -87,6 +87,7 @@ protected:
   virtual void onContactPathUpdated(const ContactInfo& contact) = 0;
   virtual void onMessageRecv(const ContactInfo& contact, uint8_t path_len, uint32_t sender_timestamp, const char *text) = 0;
   virtual void onCommandDataRecv(const ContactInfo& contact, uint8_t path_len, uint32_t sender_timestamp, const char *text) = 0;
+  virtual void onSignedMessageRecv(const ContactInfo& contact, uint8_t path_len, uint32_t sender_timestamp, const uint8_t *sender_prefix, const char *text) = 0;
   virtual uint32_t calcFloodTimeoutMillisFor(uint32_t pkt_airtime_millis) const = 0;
   virtual uint32_t calcDirectTimeoutMillisFor(uint32_t pkt_airtime_millis, uint8_t path_len) const = 0;
   virtual void onSendTimeout() = 0;